Freeze prevention of a fuel cell power plant

ABSTRACT

A fuel cell power plant comprises a water circulation passage ( 5 ) which circulates the cooling water in a fuel cell stack ( 4 ). Sodium hydroxide which lowers the melting point of water is mixed with water in the water circulation passage ( 5 ) to prevent freezing of the water in the water circulation passage ( 5 ). Electrodes ( 11, 12 ) are disposed in the water. When the fuel cell stack ( 4 ) is running, a voltage is applied between the electrodes ( 11, 12 ) such that the positive electrode ( 11 ) attracts sodium ions, thereby removing sodium ions from the cooling water supplied to the fuel cell stack ( 4 ). When the fuel cell stack ( 4 ) stops running, sodium ions which were attracted to the positive electrode ( 11 ) are made to diffuse into the cooling water by ceasing to apply the voltage to the electrodes ( 11, 12 ).

FIELD OF THE INVENTION

[0001] This invention relates to prevention of water freezing in a fuelcell power plant.

BACKGROUND OF THE INVENTION

[0002] Many fuel cell power plants mounted in vehicles use a stack ofpolymer electrolyte fuel cells (PEFC). In this type of fuel cell, anelectrolyte membrane must always be maintained in the wet state, so itis essential to supply it with Water. It is also necessary to humidifyfuel and air supplied to the polymer electrolyte fuel cell, and water isfurther used for cooling of the power plant.

[0003] Therefore, a considerable amount of water is used inside thepower plant. If the outside temperature of the power plant drops belowfreezing point when it is not operating, this water will freeze, soundesirable effects may occur, i.e., the electrolyte membrane in thefuel cell stack may be damaged, or the startup time of the power plantmay be lengthened.

[0004] Regarding this problem, Tokkai 2000-315514 published by theJapanese Patent Office in 2000 discloses an antifreeze apparatus for afuel cell power plant. The apparatus collects water in a water passageof the power plant into a tank when the power plant terminates operationat low temperature, and supplies high temperature steam to the waterpassage when the power plant starts again to thaw the frozen waterremaining in the water passage.

SUMMARY OF THE INVENTION

[0005] However, this prior art apparatus consumes a large amount ofenergy to thaw the frozen water, and the construction of the power plantis complicated by the water collecting mechanism.

[0006] Tokkai 2001-15139 published by the Japanese Patent Office in 2001discloses an apparatus which adds an antifreeze such as methanol to thewater tank so that the water required to warm up the fuel and oxidizingagent in the fuel cell power plant does not freeze. When the fuel oroxidizing agent is humidified using the water in this water tank, thewater is heated by a heater to vaporize the antifreeze before ithumidifies the fuel or oxidizing agent, so that antifreeze which wouldinterfere with power generation by the fuel cell does not mix with thefuel or oxidizing agent. The vaporized antifreeze is then liquefied bycooling in a cooler, and is recirculated to the water tank.

[0007] However, in this apparatus, the antifreeze is limited to thewater which humidifies the fuel and oxidizing agent. This apparatusheats the antifreeze to vaporize it, but if it is used to preventfreezing of the large amount of recirculated water such as cooling waterin the power plant, a large amount of energy is consumed to vaporize thewater.

[0008] It is therefore an object of this invention to prevent freezingof water in a fuel cell power plant with a low energy consumption.

[0009] In order to achieve the above object, this invention provides anantifreeze apparatus for a water circulation passage formed in a fuelcell power plant which generates power by a fuel cell stack. Theapparatus comprises an antifreeze release/recovery mechanism functioningto release antifreeze into the water circulation passage and recover theantifreeze in the water circulation passage, a sensor which detectswhether or not the fuel cell stack is operating, and a programmablecontroller.

[0010] The programmable controller is programmed to control therelease/recovery mechanism to release the antifreeze into the watercirculation passage when the fuel cell stack is not operating, andcontrol the release/recovery mechanism to recover the antifreeze whichhas diffused into the water circulation passage when the fuel cell stackis operating.

[0011] This invention also provides an antifreeze apparatus comprisingan antifreeze release/recovery mechanism functioning to releaseantifreeze into the water circulation passage when a water temperatureof the water circulation passage is not higher than a predeterminedtemperature, and recover the antifreeze in the water circulation passagewhen the water temperature of the water circulation passage is higherthan the predetermined temperature.

[0012] The details as well as other features and advantages of thisinvention are set forth in the remainder of the specification and areshown in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic cross-sectional view of a water circulationapparatus for a fuel cell power plant according to this invention.

[0014]FIG. 2 is a flowchart describing an antifreeze diffusion andrecovery routine executed by a controller according to this invention.

[0015]FIGS. 3A, 3B are timing charts describing the relation betweenantifreeze concentration during circulation of water, and output voltageof the fuel cell, according to this invention.

[0016]FIG. 4 is similar to FIG. 1, but showing a second embodiment ofthis invention.

[0017]FIG. 5 is similar to FIG. 1, but showing a third embodiment ofthis invention.

[0018]FIG. 6 is similar to FIG. 1, but showing a fourth embodiment ofthis invention.

[0019]FIG. 7 is similar to FIG. 1, but showing a fifth embodiment ofthis invention.

[0020]FIG. 8 is similar to FIG. 1, but showing a sixth embodiment ofthis invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Referring to FIG. 1 of the drawings, a fuel cell power plant forvehicles generates power from a fuel cell stack 4 made of polymerelectrolyte fuel cells. The fuel cell stack 4 generates power by areaction between oxygen in air supplied from an air passage 6 to acathode 2, and hydrogen in hydrogen-rich gas supplied from a fuelpassage 7 to an anode 3. The power generated by the fuel cell stack 4 issupplied to an electric motor 23. It is also used to charge a secondarybattery 14 via an electrical circuit, not shown.

[0022] The fuel cell stack 4 comprises a cooling water passage 1 forabsorbing heat due to power generation.

[0023] A water circulation passage 5 is connected to the cooling waterpassage 1. The water circulation passage 5 is a closed circuit whichpasses through the cooling water passage 1. A pump 8, tank 9, three-wayvalve 17 and ion removal filter 10 are provided midway in the watercirculation passage 5.

[0024] The pump 8 pressurizes water in the water circulation passage 5downstream of the cooling water passage 1 and supplies it to the tank 9.The tank 9 is the sealed type, so that the interior of the tank 9 ispressurized by the operation of the pump 8, and water in the tank 9 iscirculated through the water circulation passage 5 under this pressure.The ion removal filter 10 is a filter which removes impurities dissolvedin the cooling water circulating through the water circulation passage5, and the three-way valve 17 is a valve which selectively leads waterfrom the water circulation passage 5 to the ion removal filter 17 or abypass passage 19 which bypasses the ion removal filter 17. Thechange-over of the three-way valve 17 is performed according to achange-over signal from a controller 18.

[0025] An antifreeze release/recovery mechanism 40 is installed in thetank 9. The antifreeze release/recovery mechanism 40 compriseselectrodes 11, 12 installed in the water in the tank 9, the switch 13and the secondary battery 14.

[0026] The antifreeze is sodium hydroxide which is previously mixed withwater in the tank 9. The sodium hydroxide is present in the ionic statein the water. When a voltage is applied from the secondary battery 14via the switch 13 so that the electrode 12 becomes a positive electrodeand the electrode 11 becomes a negative electrode, sodium ions in thewater, which are cations accumulate around the negative electrode 11.Due to this accumulation, the antifreeze release/recovery mechanism 40stores antifreeze. On the other hand, when application of the voltage tothe electrodes 11, 12 is halted, the sodium ions mix with therecirculated water, and are released to the water circulation passage 5.In this state, if a voltage is again applied to the electrodes 11, 12,the sodium ions in the recirculated water again accumulate around theelectrode 11, so that the substance flowing through the watercirculation passage 5 is pure water.

[0027] The switch 13 respectively connects the positive electrode of thesecondary battery 14 to the electrode 12 and the negative electrode ofthe secondary battery to the electrode 11 according to a connectionsignal from the controller 18. These connections are also brokenaccording to an interruption signal from the controller 18.

[0028] The antifreeze, sodium hydroxide, is present in the ionic statein the water. Therefore, when power is supplied to the electrodes 11,12, sodium ions which are anions are attracted to the negative electrode11.

[0029] When a voltage is applied to the electrodes 11, 12, hydrogen gasis produced on the surface of the negative electrode 11 11 and oxygengas is produced on the surface of the positive electrode 12. A duct 15for discharging hydrogen gas and a duct 16 for discharging oxygen gasare connected to the tank 9, and the hydrogen gas and oxygen gasproduced are released outside the tank 9, so that these gases do notincrease the pressure inside the tank 9. One end of each of the ducts15, 16 respectively projects inside the tank 9 so as to cover theelectrodes 11, 12 from above, and the other end has an L-shaped bendinside the tank 9 which opens onto the outside of the tank 9.

[0030] The controller 18 operates the three-way valve 17 and switch 13according to the running state of the fuel cell stack 4. Herein, therunning state of the fuel cell stack 4 may be divided into running,startup and stop states. Running of the fuel cell stack 4 can bedetected from the output current from the fuel cell stack 4 using anammeter 20. Startup of the fuel cell stack 4 can be detected from thecurrent supplied from the secondary battery 14 to a startup circuit 21comprising a compressor or heater which supplies air to the fuel cellstack 4, using an ammeter 22.

[0031] When the current values detected by the ammeter 20, 22 are bothzero, the fuel cell stack 4 has stopped running, when the current valuedetected by the ammeter 20 is zero but the current detected by theammeter 22 is not zero, the fuel cell stack 4 is starting up, and whenthe currents detected by the ammeter 20, 22 are both not zero, the fuelcell stack 4 is running. Herein, the running of the fuel cell stack 4means that the fuel cell stack 4 is generating electric power.

[0032] When the fuel cell stack has stopped, the controller 18interrupts the switch 13 so that a voltage is not applied to theelectrodes 11, 12. When a voltage is not applied to the electrodes 11,12, the cathode 11 loses its power to attract sodium ions, and sodiumions which were staying around the cathode 11 diffuse through thecooling water. When the running of the fuel cell stack 4 stops, the pump8 continues running for a short time after the switch 13 is interrupted,and diffusion of sodium ions into the cooling water is promoted. Thepump 8 runs on a supply current from the secondary battery 14 accordingto a running signal from the controller 18. At this time, the three-wayvalve 17 is held in a position which leads cooling water to the bypasspassage 19. Due to this operation, antifreeze diffuses throughout all ofthe water circulation passage 5 and cooling water passage 1 in the fuelcell stack 4, so that water in the water circulation passage 5 andcooling water passage 1 does not easily freeze even after the pump 8 hasstopped running. Also, as the cooling water does not pass through theion removal filter 10, sodium ions are not removed by the ion removalfilter 10.

[0033] When the fuel cell stack 4 is starting up, the controller 18 runsthe pump 8 and applies a voltage to the electrodes 11, 12. Due to thisoperation, cooling water circulates through the water circulationpassage 5, sodium ions in the cooling water are attracted to thenegative electrode 11 in the tank 9 during circulation, and build uparound the negative electrode 11. Therefore, antifreeze can be recoveredfrom the cooling water in a short time. At this time also, the three-wayvalve 17 is held in the position which leads cooling water to the bypasspassage 19.

[0034] When the fuel cell stack 4 shifts from startup to running, thecontroller 18 continues to run the pump 8 and apply a voltage to theelectrodes 11, 12, and the three-way valve 17 changes over to a positionwhich leads cooling water to the ion removal filter 10. In this state,sodium ions have already built up around the cathode 11, and are notcontained in the cooling water. Therefore, the ion removal filter 10does not decrease the sodium ions.

[0035] By performing operation so that sodium ions in the cooling waterare recovered by applying a voltage to the electrodes 11, 12 when thefuel cell stack 4 starts up, the cooling water passes through the ionremoval filter 10 only when the fuel cell stack 4 is running, andremoval of ions by the ion removal filter 10 is suppressed to theminimum. This operation prevents loss of antifreeze and has a desirableeffect on maintaining the performance of the ion removal filter 10.

[0036] Next, referring to FIG. 2, the routine executed by the controller18 to perform the above control will be described by a flowchart. Thisroutine is executed as an interval of ten milliseconds irrespective ofrunning, startup and stop of the fuel cell stack 4.

[0037] First, in a step S1, the controller 18 determines whether or notthe detection current value of the ammeter 20 is zero. When thedetection current value of the ammeter 20 is zero, in a step S2, it isdetermined whether or not the detection current value of the amateur 22is zero.

[0038] When the detection current value of the ammeter 22 in the step S2is zero, the fuel cell stack 4 is in the stop state. In this case, in astep S3, it is determined whether are not a predetermined time haselapsed from when the detection current value of the ammeter 20 becamezero. This determination is a value for determining whether or not thefuel cell stack 4 has just stopped generating power. The predeterminedtime is preferably set equal to several minutes.

[0039] If the predetermined time has not elapsed, in a step S6, the pump8 runs, the three-way valve 17 is held in the position which leadscooling water to the bypass passage 19, and the switch 13 is switchedoff so that a voltage is not applied to the electrodes 11, 12. Due tothis operation, sodium ions which were staying around the electrode 11diffuse through the water circulation passage 5 and cooling waterpassage 1 in the fuel cell stack 4.

[0040] After the predetermined time has elapsed, in a step S7, therunning of the pump 8 is stopped, the three-way valve 17 is held in aposition which leads cooling water to the bypass passage 19, and theswitch 13 is held in the off state.

[0041] On the other hand, when in the step S2, the detection currentvalue of the ammeter 22 is not zero, it means that the power plant isstarting up. In this case, the controller 18, in a step S6, runs thepump 8, holds the three-way valve in the position which leads coolingwater to the bypass passage 19, and switches on the switch 13 so that avoltage is applied to the electrodes 11, 12. Due to this operation, whenthe power plant starts up, cooling water is circulated through the watercirculation passage 5, sodium ions are attracted to the negativeelectrode 11 in the in the tank 9 midway along the water circulationpassage 5 and antifreeze is recovered from the cooling water.

[0042] When the detection current value of the ammeter 20 is not zero inthe step S1, the fuel cell stack 4 has already started running. In thiscase, in the step S4, the controller 18 runs the pump 8, holds thethree-way valve 17 in the position which leads cooling water to the ionremoval filter 10, and switches on the switch 13 so that a voltage isapplied to the electrodes 11, 12.

[0043] Due to this operation, pure water is supplied from the watercirculation passage 5 to the cooling water passage 1 inside the fuelcell stack 4.

[0044] After performing any one of the steps S4-S7, the controller 18terminates the routine.

[0045] Referring to FIGS. 3A, 3B, while the fuel cell stack 4 hasstopped running, the antifreeze concentration in the cooling water ismaintained at a high level, so freezing of cooling water is prevented.When the fuel cell stack 4 starts up, the negative electrode 11 recoversantifreeze from the cooling water such that the antifreeze in thecooling water, i.e., the concentration of sodium ions, drops sharply.Therefore, when running starts after startup, the antifreezeconcentration in the cooling water is maintained at zero. When the fuelcell stack 4 stops running, sodium ions which were attracted to thenegative electrode 11 are released into the cooling water, so the sodiumion concentration in the cooling water increases sharply. Hence, releaseand recovery of antifreeze in the cooling water can be performed asrequired according to the running state of the fuel cell stack 4, andfreezing of the power plant can be prevented without consuming a largeamount of energy. Also, the time required for recovery of antifreeze canbe adjusted by increasing or decreasing the voltage applied to theelectrodes 11, 12.

[0046] In this embodiment, sodium hydroxide was used as the antifreeze,but any organic or inorganic substance which dissociates in water andreduces the freezing point of water may also be used as the antifreeze.

[0047] In this embodiment, the switch 13 connected the secondary battery14 to the electrodes 11, 12, however a condenser may also be usedinstead of the secondary battery 14. Also, in a power plant comprisingplural fuel cell stacks, current can also be supplied to the electrodes11, 12 by the switch 13 from another fuel cell stack which has alreadystarted supplying power.

[0048] Further, as a more primitive example, the fuel cell stack 4 maybe connected to the electrodes 11, 12 via the switch 13, and a voltageapplied to the electrodes 11, 12 using the power generated by the fuelcell stack 4 which has started running. In this case, a voltage cannotbe applied to the electrodes 11, 12 while the fuel cell stack 4 isstarting up, but after the fuel cell stack 4 has started generatingpower, antifreeze can be recovered in a short time.

[0049] In this embodiment, the ion removal filter 10 and three-way valve17 were provided, but these may be omitted. In this case, the operationof the three-way valve 17 in the steps S4-S7 is unnecessary.

[0050] Next, a second embodiment of this invention will be describedreferring to FIG. 4.

[0051] In this embodiment, an electromagnet 20 is disposed in the tank 9instead of the electrodes 11, 12 as the antifreeze release/recoverymechanism 40. The electromagnet 20 is connected to the secondary battery14 via the switch 13. Also, fine particles of a magnetic material whichdo not dissolve in water are used instead of sodium hydroxide as theantifreeze. The structure of the fine particles is such that theydecrease the freezing point due to the phase change of water resultingfrom mutual interaction with water. Examples of fine particles havingthis property are porous fine particles, fine particles having surfaceimperfections, and fine particles coated with material havinghydrophilic groups and hydrophobic groups.

[0052] Herein, magnetic fine particles having iron as the main componentand a diameter of one micrometer (μm) are used as the antifreeze.

[0053] When the fuel cell stack starts up, the pump 8 runs, the switch13 is switched on, and the electromagnet 20 is energized by thesecondary battery 14. Due to the magnetic force, when the magneticparticles in the cooling water circulating in the water circulationpassage 5 pass through the tank 9, the energized electromagnet 20attracts the magnetic fine particles dispersed in the cooling water. Asa result, the fine magnetic particles are removed from the coolingwater. The switch 13 also remains in the on state and the fine magneticparticles remain attracted to the surface of the electromagnet 20 whenthe fuel cell stack 4 is running. Hence, diffusion and dispersion of theantifreeze can be performed using the fine particles of magneticmaterial which decrease the freezing point of water and theelectromagnet 20 instead of an antifreeze which has dissociatingproperties and the electrodes 11, 12.

[0054] In this embodiment, the antifreeze diffusion and recovery routineexecuted by the controller 18 is identical to the routine of FIG. 2 inthe first embodiment.

[0055] Next, a third embodiment of this invention will be describedreferring to FIG. 5.

[0056] In this embodiment, sodium chloride which is a dissociatingsubstance, is used as the antifreeze, and a polymer aggregate 30 havinga temperature response is filled in the tank 9 instead of the electrodes11, 12 of the first embodiment as the antifreeze release/recoverymechanism 40.

[0057] The temperature response polymer uses a molecular matrix obtainedby treating a polymer material having n-isopropyl acrylamide as its maincomponent, by the molecular template polymerization method. Thistemperature response polymer has a property whereby its high-orderstructure changes according to the temperature, i.e., it expands at lowtemperature and contracts at high temperature.

[0058] When the cooling water is at low temperature, sodium chloride isdissolved as ions in the cooling water. The molecular matrix of thetemperature response polymer which forms the aggregate 30 in the tank 9expands, so sodium chloride ions pass freely through it. When thetemperature of the cooling water is raised, the molecular matrixshrinks, so sodium chloride ions in the cooling water flowing into thetank 9 are trapped, and the aggregate 30 gels.

[0059] In this way, the aggregate 30 collects the sodium chloride ionswhich constitute the antifreeze, from the cooling water, and releasesthem into the cooling water according to the temperature.

[0060] This temperature phase transition takes place at a boundary ofapproximately 30 degrees centigrade (° C.). When the cooling watertemperature falls below this temperature, the molecular matrix of thetemperature response polymer of the aggregate 30 expands again, sosodium chloride ions held therein are again released into the coolingwater.

[0061] In this embodiment, electricity is not supplied to the aggregate30, so the energy consuming for antifreeze recovery and release isbasically zero. Further, the switch 13 is not used, so it is unnecessaryto control the switch 13, and the controller 18 need only control therunning of the pump 8 and the change-over of the three-way valve 17.

[0062] Other materials may also be used instead of the molecular matrixof n-isopropyl acrylamide as the temperature response polymer.Specifically, a polymer or gel having a compound structure of thetemperature response polymer and another polymer, or a material whereinthe temperature response polymer is crafted onto a fine particlesurface, may be used.

[0063] Next, a fourth embodiment of this invention will be describedreferring to FIG. 6.

[0064] In this embodiment, the tank 9 is divided into chambers 9A, 9B bya permeable membrane 50 instead of the electrodes 11, 12 of the firstembodiment, as the antifreeze release/recovery mechanism 40. The inletand outlet of the tank 9 both face the chamber 9A. A valve 51 andpressurizing mechanism 52 are further provided between the tank 9 andthree-way valve 17. Herein, an auxiliary pump may be used as thepressurizing mechanism 52. The permeable membrane 50 comprises forexample a membrane made of polyamide or cellulose. The antifreezecomprises a material having molecular dimensions such that it passesthrough the permeable membrane 50 above a certain pressure, but does notpass through the permeable membrane 50 below the certain pressure.

[0065] In this embodiment, cooling water is circulated in the watercirculation passage 5 under a predetermined pressure due to the runningof the pump 8 when the fuel cell stack 4 is starting up or running. Dueto this pressure, antifreeze passes from the chamber 9A to the chamber9B, and antifreeze in the cooling water is recovered to the chamber 9B.

[0066] The rate of recovering antifreeze depends on the pressure ofcooling water. When the fuel cell stack 4 starts up, the pump 8 runs,and the pressure in the chamber 9A can be raised by closing the valve 51so as to increase the antifreeze recovery rate. Alternatively, theantifreeze recovery rate can also be increased by opening the valve 51,and raising the pressure of recirculating cooling water by thepressurizing mechanism 52. It is not absolutely necessary to provideboth the valve 51 and pressurizing mechanism 52, and the antifreezerecovery rate may be varied by providing only one of them.

[0067] Next, a fifth embodiment of this invention will be describedreferring to FIG. 7.

[0068] In this embodiment, an electrodialyzer 62 is disposed between apositive electrode 60 and a negative electrode 61 as the antifreezerelease/recovery mechanism 40. The antifreeze is a substance havingdissociating groups such as a salt, alcohol or sugar.

[0069] The electrodialyzer 62 has plural chambers 62A-62E partitioned bytwo anion exchange membranes 69 and two cation exchange membranes 68which are disposed alternately.

[0070] One of the cation exchange membranes 68 is disposed nearest thenegative electrode 61 and one of the anion exchange membranes 69 isdisposed nearest the positive electrode 60.

[0071] The water supply passage 5 is connected to the electrodialyzer 62so that cooling water in the water circulation passage 5 passes throughthe chambers 62B, 62D. The cooling water is not supplied to the chambers62A, 62C, 62D.

[0072] When the cooling water led to the electrodialyzer 62 passesthrough the chambers 62B, 62D, anions in the cooling water are attractedto the positive electrode 60, and cations in the cooling water areattracted to the negative electrode 61 due to the potential differencebetween the positive electrode 60 and negative electrode 61. Therefore,anions flow from the chamber 62B to the chamber 62A via the anionexchange membrane 68. Likewise, cations flow to the chamber 62C via thecation exchange membrane 69. From the chamber 62D, anions flow to thechamber 62C via the anion exchange membrane 69 and cations flow to thechamber 62E via the cation exchange membrane 68.

[0073] An antifreeze circulation passage 63 comprising an antifreezetank 64 and antifreeze pump 65, is formed via the chambers 62A, 62C aspart of the antifreeze release/discharge mechanism 40. Further, a valve67A is provided which supplies antifreeze in the antifreeze tank 64 tothe water circulation passage 5 upstream of the electrodialyzer 62, anda valve 67B is provided which supplies antifreeze to the watercirculation passage 5 downstream of the electrodialyzer 62.

[0074] When the fuel cell stack 4 has stopped, a voltage is not suppliedto the electrode 60, 61, the valves 67A, 67B are opened, and antifreezein the antifreeze tank 64 is released to the water circulation passageby running the antifreeze pump 65. As a voltage is not supplied to theelectrodes 60, 61, the cooling water which passes through theelectrodialyzer 62, passes through the chambers 62B, 62D withoutseparation of antifreeze.

[0075] When the fuel cell stack 4 is starting up, the valves 67A, 67Bare closed, and the switch 13 is switched on to apply a voltage to theelectrodes 60, 61. As a result, anions pass from the chamber 62B intothe chamber 62A, and cations pass into the chamber 62C. Also, cationspass from the chamber 62D into the chamber 62E and anions pass into thechamber 62C. As a result, antifreeze is separated from the coolingwater, and is recovered by the antifreeze tank 64 via the antifreezecirculation passage 63. Pure water from which the antifreeze has beenremoved is supplied from the chambers 62B, 62D to the water circulationpassage 5. After the antifreeze is recovered by the antifreeze tank 64,the fuel cell stack 4 is started up. There is no need to apply a voltageto the electrodes 60, 61 while the fuel cell stack 4 is running.

[0076] When the capacity of the antifreeze tank 64 is large, theantifreeze pump 65 may be omitted. Conversely, by providing theantifreeze pump 65, the antifreeze tank 64 can be made compact.

[0077] When the fuel cell stack 4 has stopped, when the antifreeze pump65 is running, a negative voltage is applied to the positive electrode60 and a positive voltage is applied to the negative electrode 62,antifreeze can be supplied from the chambers 62A, 62B, 62E to thecooling water in the chambers 62B, 62D.

[0078] If this operation is applied for supplying antifreeze to thewater circulation passage 5, the valves 67A, 67B may be omitted.

[0079] In this embodiment, five chambers 62A-62E were formed using twoanion exchange membranes 69 and two cation exchange membranes 68, butthe number of chambers of the electrodialyzer 62 may be set as desiredprovided that the total number of ion exchange membranes is even, i.e.,the number of chambers is odd.

[0080] Next, a sixth embodiment of this invention will be describedreferring to FIG. 8.

[0081] In this embodiment, the release/recovery mechanism 40 has thefollowing construction.

[0082] Specifically, the tank 9 is divided into two chambers 9A, 9B inthe same way as in the fourth embodiment. The water circulation passage5 is connected to the chamber 9A. A second water circulation passage 73is connected to the chamber 9B. In the second water circulation passage73, a pump 72 and an antifreeze primary release/recovery mechanism 71are provided.

[0083] The antifreeze primary release/discharge mechanism 71 has anidentical construction to the release/discharge mechanisms 40 in any ofthe first-fifth embodiments. A material identical to that of the fourthembodiment is used as antifreeze.

[0084] When the fuel cell stack 4 has stopped, the antifreeze primaryrelease/recovery mechanism 71 supplies antifreeze to the water in thesecond water circulation passage 73. As a result, a concentrationdifference arises between the antifreeze in the chambers 9A, 9B, andantifreeze passes from the chamber 9B to the chamber 9A via thepermeable membrane 50.

[0085] On the other hand, when the fuel cell stack 4 is starting up, theantifreeze primary release/recovery mechanism 71 recovers antifreezewhich has diffused into the second water circulation passage 73. As aresult, as the antifreeze concentration in the chamber 9B is less thanin the chamber 9A, antifreeze flows from the chamber 9A to the chamber9B via the permeable membrane 50. As a result of this operation, theantifreeze in the water circulation passage 5 is also recovered by theantifreeze primary release/recovery mechanism 71. After recovery ofantifreeze in the water circulation passage 5 is complete, running ofthe fuel cell stack 4 is performed.

[0086] In this embodiment, the antifreeze primary release/recoverymechanism 71 was provided in the second water circulation passage 73instead of the circulation passage 5, so the circulation passage 5 isnot affected by the pressure loss due to the antifreeze primaryrelease/recovery mechanism 71. Therefore, there is the advantage thatthe pressure of the cooling water is not affected by antifreezerelease/recovery operation.

[0087] The contents of Tokugan 2002-32392, with a filing date of Feb. 8,2002 in Japan, are hereby incorporated by reference.

[0088] Although the invention has been described above by reference tocertain embodiments of the invention, the invention is not limited tothe embodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the above teachings.

[0089] For example, the above embodiments concern prevention of freezingof cooling water in the fuel cell stack 4. However, this invention isnot limited to the cooling water in the fuel cell stack 4, and may alsobe applied to prevent any recirculated water used by a fuel cell powerplant from freezing, including a recirculation circuit which circulateswater to humidify fuel or an oxidizing agent as disclosed in theaforesaid Tokkai 2001-15139.

INDUSTRIAL FIELD OF APPLICATION

[0090] As described above, according to this invention, release ofantifreeze into the water circulation passage and recovery of antifreezefrom the water circulation passage can be repeatedly performed with asmall energy consumption according to the running conditions of the fuelcell stack. Therefore, this invention has a particularly desirableaffect when it is applied to a fuel cell power plant for driving avehicle.

The embodiments of this invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. An antifreeze apparatusfor a water circulation passage (5) formed in a fuel cell power plantwhich generates power by a fuel cell stack (4), characterized by: anantifreeze release/recovery mechanism functioning to release antifreezeinto the water circulation passage (5) and recover the antifreeze in thewater circulation passage (5); first means (18) for controlling therelease/recovery mechanism (40) to release the antifreeze into the watercirculation passage (5) when the fuel cell stack (4) is not operating(S4); and second means (18) for controlling the release/recoverymechanism (40) to recover the antifreeze which has diffused into thewater circulation passage (5) when the fuel cell stack (4) is operating(S6, S7).
 2. The antifreeze apparatus as defined in claim 1, wherein theapparatus further comprises a sensor (20) which detects whether or notthe fuel cell stack (4) is operating, and the first means (18) and thesecond means (18) are provided in the form of a programmable controller(18) programmed to control the release/recovery mechanism (40) torelease the antifreeze into the water circulation passage (5) when thefuel cell stack (4) is not operating (S4), and control therelease/recovery mechanism (40) to recover the antifreeze which hasdiffused into the water circulation passage (5) when the fuel cell stack(4) is operating (S6, S7).
 3. The antifreeze apparatus as defined inclaim 1 or claim 2, wherein the antifreeze comprises a substance whichdissociates into an anion and a cation.
 4. The antifreeze apparatus asdefined in claim 3, wherein the release/recovery mechanism (40)comprises a pair of facing electrodes (11, 12) in water, a power supply(40) which applies a voltage between the electrodes (11, 12), and aswitch (13) which controls application of the voltage between theelectrodes (11, 12) by the power supply (14), and the programmablecontroller (18) is further programmed to control the switch (13) so thatthe voltage is not applied between the electrodes (11, 12) by the powersupply (14) when the fuel cell stack (4) is not operating (S6, S7), andcontrol the switch (13) so that the voltage is applied between theelectrodes (11, 12) by the power supply (14) when the fuel cell stack(4) is operating (S4).
 5. The antifreeze apparatus as defined in claim4, wherein the apparatus further comprises a sensor (22) which detectswhether or not the fuel cell stack (4) is starting up, and thecontroller (18) is further programmed to control the switch (13), whenthe fuel cell stack (4) is starting up, to apply the voltage between theelectrodes (11, 12) by the power supply (14) (S5).
 6. The antifreezeapparatus as defined in claim 5, wherein the fuel cell power plantfurther comprises a start-up circuit (21) which operates by a powersupplied from the power supply (14) for starting up the fuel cell stack(4), and the sensor (22) which detects whether or not the fuel cellstack (4) is starting up, comprises an ammeter (22) which detects acurrent supplied from the power supply (14) to the start-up circuit(21).
 7. The antifreeze apparatus as defined in claim 4, wherein theapparatus further comprises passages (15, 16) which discharge gasproduced by the electrodes (11, 12) due to the application of thevoltage between the electrodes (11, 12) by the power supply (14).
 8. Theantifreeze apparatus as defined in claim 2, wherein the apparatusfurther comprises a pump (8) which recirculates water in the watercirculation passage (5), and the controller (18) is further programmedto control the pump (8) to stop running when the fuel cell stack (4) isnot operating and a predetermined time has elapsed from when the fuelcell stack (4) stopped operation, while controlling the pump (8) to keeprunning in other cases (S4-S6).
 9. The antifreeze apparatus as definedin claim 3, wherein the release/recovery mechanism (40) comprises ananion exchange membrane (69), a first chamber (62A) facing the anionexchange membrane (69), a cation exchange membrane (68), a secondchamber (62E) facing the cation exchange membrane (68), a third chamber(62B-62D) facing the anion exchange membrane (69) and the cationexchange membrane (68), a positive electrode (60) which attracts anionscontained in the water in the third chamber (62B-62D) via the anionexchange membrane (69), a negative electrode (61) which attracts cationscontained in the water in the third chamber (9B-9D) via the cationexchange membrane (68), and a power supply (14) which applies a voltagebetween the positive electrode (11) and the negative electrode (12). 10.The antifreeze apparatus as defined in claim 2, wherein the sensor (20,22) which detects whether or not the fuel cell stack (4) is operatingcomprises an ammeter (20) which detects an output current from the fuelcell stack (4).
 11. The antifreeze apparatus as defined in claim 1 orclaim 2, wherein the antifreeze contains fine particles of a magneticmaterial which decreases the melting point of water, and therelease/discharge mechanism (40) comprises an electromagnet (20) whichexerts a magnetic force on the water circulating through the watercirculation passage (5), and a power supply (14) which supplies anenergization current to the electromagnet (20).
 12. The antifreezeapparatus as defined in claim 1 or claim 2, wherein the release/recoverymechanism (40) comprises a permeable membrane (50) facing the water inthe water circulation passage (5), a chamber (9B) partitioned from thewater in the water circulation passage (5) by the permeable membrane(50), and a pressurizing apparatus (8, 51, 52) which pressurizes thewater in the water circulation passage (5), and the antifreeze comprisesa material which passes through the permeable membrane (50) depending ona pressure.
 13. The antifreeze apparatus as defined in claim 12, whereinthe release/recovery mechanism (40) further comprises a second watercirculation passage (73) connected to the chamber (9B), a primaryrelease/recovery mechanism (71) functioning to release the antifreezeinto the second water circulation passage (73) and recover theantifreeze diffused in the second water circulation passage (73), and apump (72) which pressurizes the water in the second water circulationpassage (73).
 14. The antifreeze apparatus as defined in any one ofclaim 4, wherein the power supply (14) comprises a secondary battery(14) which is charged by an electric power generated by the fuel cellstack (4).
 15. An antifreeze apparatus for a water circulation passage(5) formed in a fuel cell power plant which generates power by a fuelcell stack (4), characterized by: an antifreeze release/recoverymechanism (30) functioning to release antifreeze into the watercirculation passage (5) when a water temperature of the watercirculation passage (5) is not higher than a predetermined temperature,and recover the antifreeze in the water circulation passage (5) when thewater temperature of the water circulation passage (5) is higher thanthe predetermined temperature.
 16. The antifreeze apparatus as definedin claim 15, wherein the antifreeze release/recovery mechanism (30)comprises a molecular matrix aggregate (30) of a polymer material whichexpands when the water temperature is not higher than the predeterminedtemperature, and contracts when the water temperature is higher than thepredetermined temperature, the water circulation passage (5) beingarranged to pass through the aggregate (30).